The present invention relates to a bone screw which comprises a device for electrostimulation.
It is known that the growth of tissue can be promoted by a low-frequency, essentially sinusoidal alternating current (Method according to Kraus and Lechner). It is known, for applying the alternating current to the region of the tissue to be stimulated, to implant an electric coil (xe2x80x9creceiving coilxe2x80x9d) the terminals of which being coupled to electrodes (xe2x80x9ctissue electrodesxe2x80x9d) applied to the tissue region to be stimulated. An electric voltage is induced in the receiving coil by means of an external primary coil coupled to an oscillation generator, said voltage being applied via said electrodes to the tissue region to be attended (DE-A-31 32 488).
The receiving coil may be housed in an intramedullary nail, the surface of which being provided with tissue electrodes (DE-A-31 32 488). It is further known, to couple the terminals of a receiving coil by snap-fastener-like connecting means to bone screws serving for osteosynthesis or only as electrodes (DE-A-39 42 735). A bone screw is also know which contains a rod-shaped permanent magnet.
The present invention is based on the task, to provide a bone screw with a device for electrostimulation which is easily to apply, safe in use and versatile.
A femur head prostesis is known from document EP-A-0 781 532 comprising a shaft which has an interior cavity and holes which lead from the cavity to the exterior. The upper end of the shaft has a threaded hole for screwing-on an extraction tool, said hole being adapted to be closed by a drive-in screw. The cavity serves for receiving spongiosa into which the tissue surrounding the shaft should grow through the holes. To enhance the growing-in of the tissue, the drive-in screw is replaced by an electrifying device. This device has a hexagonal head, an adjacent thread fitting into the threaded hole, and a shaft protruding into the cavity. The shaft contains a receiving coil connected to electrodes so that a low-frequency electric current stimulating the growth of tissue can be induced in the cavity. This electrifying device is neither intended nor suited to be used as bone screw, because it would not be able to withstand the stresses to which a bone screw is subjected. Further the machine-screw thread, which must match the threaded hole, is not suited for a bone screw because of its configuration and position.
Dental implants are known from documents U.S. Pat. Nos. 5,292,252 and 5,725,377 which have a shaft with an external thread and a interior threaded hole at the upper end for fixing a tooth prostesis. A stimulator cap containing a battery is screwed into the threaded hole. In one embodiment, the battery is connected to a coil positioned in the shaft to produce a magnetic d.c. field in the surrounding at the implant. In other embodiments, the battery is connected to the shaft and to an electrode positioned on the front side of the stimulator cap. These dental implants are not suited for use as bone screws as they are needed for fixing osteosythesis plates or for connecting bone fragments.
An implant for electrostimulation of tissue is disclosed in document DE-A-42 30 181 which comprises a receiving coil in which a low-frequency electric voltage can be induced as described in document DE-A-31 32 488 mentioned above. The receiving coil is coupled by means of flexible leads to connectors for contacting conventional bone screws or tissue electrodes.
It is an object of the present invention to provide a device for electrostimulation of tissue by a low-frequency electric alternating current which can be used exactly like a conventional bone or cortical screw and in addition to its mechanical function produces an effective electrostimulation of the surrounding tissue.
This object is obtained according to the present invention by a bone screw comprising
a tubular shaft made of biocompatible, electrically conductive material, said shaft having a head at a first end, said head having a configuration adapted to be engaged by a turning tool;
an end-piece made of biocompatible electrically conductive material positioned near a second end of said shaft;
an electric insulation between shaft and end-piece;
a receiving coil in said shaft in which coil a low-frequency electric alternating voltage can be induced by means of an external coil, first and second terminals of said receiving coil being electrically coupled to said shaft and said end-piece, respectively; and
a bone-scew-type external thread positioned near the end of said shaft remote from said head
Advantegeous embodiments of the invention further exhibit one or several of the following features: The end-piece has a front portion with a rod-shaped extension inserted insulatedly in the second end of the shaft. Electrically insulating ring-shaped means is positioned between the extension and the shaft and an electrically insulating material is provided between the second end of the shaft and the front portion. The end-piece tapers with increasing distance from the shaft. The external thread is positioned on the shaft near the second end thereof and spaced therefrom or adjacent the second end or on the end-piece. The bone screw, a core of the receiving coil, the end-piece and the insulating material have an axial channel through which a guide wire can be put. The bone screw can comprise two parts, i. e. it comprises a tubular screw portion and an insert fitting into it, the insert comprising the end-piece and the receiving coil. Further features and benefits of the invention will become apparent from the following description.
A bone screw according to a first preferred embodiment of the invention has a hollow tubular shaft with a head at one end and a self-tapping external thread at the other end. The head has, as usual, an hexagonal hole for inserting a turning tool. An electrically insulated end-piece which tapers toward the end in form of a cone or a pointed arch is inserted in the other end. A receiving coil is located in the hollow shaft, the terminals of the coil being electrically coupled to the shaft and the end-piece. The space between the end-piece and the end of the shaft, and the interior of the shaft are filled by casting with a biocompatible insulating compound, such as an epoxy resin.
The present bone screw can be applied easily like a conventional bone screw. The connection between the receiving coil and the parts serving as electrodes, i. e. the shaft and the end-piece, is mechanically and electrically safe, because it is located within the interior of the shaft filled with insulating matter. The taper of the end-piece facilitates any later extraction. The bone screw of the invention can be used for many indications, e. g. avascular necrosis of the head of femur, osteochondrosis dissecans, vertebral ankylosis, and in combination with additional conventional bone screws for fixing an intra-articular fracture of the femur neck.